Known types of telecontrol system generally include, and as indicated diagrammatically in FIG. 1 of the accompanying drawings, a portable emitter 10 which is carried by a user, together with a receiver module 20 which is fitted in a motor vehicle 30. The emitter 10 is so designed as to generate a coded wave 10a. A "coded wave" is to be understood to mean a wave which carries information or data in the form of a control or command signal. Such a coded wave may be produced by radio transmission, light transmission, infrared transmission or ultrasonic transmission, though this list is not exhaustive.
The receiver module 20 is so designed as to detect the coded wave generated by the emitter, and to decode the latter. When the code generated by the emitter 10 corresponds to one or more predetermined codes, the receiver causes locking and unlocking of the doors of the motor vehicle 30, or the operation of various auxiliary functions of the vehicle, to be carried out.
Telecontrol systems using a coded wave, with radio transmission in particular, are of very flexible application, to the extent that firstly, the user has no need to orientate the emitter towards the vehicle in order to establish transmission, and secondly, the transmission may be established at a distance of some tens of meters from the vehicle.
Generally, in the higher quality systems, the emitter 10 has a set of touch keys which are associated with particular functions in the vehicle, for example locking of the doors, unlocking of the doors, operation of the courtesy lighting or the driving or parking lights of the vehicle, closing of the windows, and activation of an alarm. Since each function has its own touch key, the size of the portable emitter unit itself is quite large, and the presence of too many touch keys detracts from the convenience of the user.
For certain control functions, such as unlocking the doors and closing the windows remotely, a high transmission range can be a factor which is detrimental to security. Accidental touching of the touch keys of the emitter unit could in this connection cause the doors to become unlocked without the user being aware of it.
On the other hand, there are some functions, such as remote control of courtesy lights or other lights in the vehicle, which it may be convenient or desirable to operate remotely from quite a long distance away, for example for the purpose of remote inspection of the vehicle, in a parking lot for instance. Under these circumstances, a transmission range of several tens of meters is desirable.
It is thus apparent that with known conventional telecontrol systems, the compromise between convenience or use of the telecontrol system on the one hand, and security considerations on the other, leads to the choice of a transmission range of between 5 and 10 meters. This transmission range favors security at the expense of convenience.